Scientists think 'planetary pebbles' were the building blocks for the largest planets

This artist’s concept of a young star system shows gas giants forming first, while the gas nebula is present. Southwest Research Institute scientists used computer simulations to nail down how Jupiter and Saturn evolved in our own solar system. These new calculations show that the cores of gas giants likely formed by gradually accumulating a population of planetary pebbles – icy objects about a foot in diameter. Credit: NASA/JPL-Caltech

Researchers at Southwest Research Institute (SwRI) and Queen's University in Canada have unraveled the mystery of how Jupiter and Saturn likely formed. This discovery, which changes our view of how all planets might have formed, will be published in the Aug. 20 issue of Nature.

Ironically, the largest planets in the solar system likely formed first. Jupiter and Saturn, which are mostly hydrogen and helium, presumably accumulated their gasses before the solar nebula dispersed. Observations of young star systems show that the gas disks that form planets usually have lifetimes of only 1 to 10 million years, which means the gas giant planets in our solar system probably formed within this time frame. In contrast, the Earth probably took at least 30 million years to form, and may have taken as long as 100 million years. So how could Jupiter and Saturn have formed so quickly?

The most widely accepted theory for gas giant formation is the so-called core accretion model. In this model, a planet-sized core of ice and rock forms first. Then, an inflow of interstellar gas and dust attaches itself to the growing planet. However, this model has an Achilles heel; specifically, the very first step in the process. To accumulate a massive atmosphere requires a solid core roughly 10 times the mass of Earth. Yet these large objects, which are akin to Uranus and Neptune, had to have formed in only a few million years.

In the standard model of planet formation, rocky cores grow as similarly sized objects accumulate and assimilate through a process called accretion. Rocks incorporate other rocks, creating mountains; then mountains merge with other mountains, leading to city-sized objects, and so on. However, this model is unable to produce planetary cores large enough, in a short enough period of time, to explain Saturn and Jupiter.

"The timescale problem has been sticking in our throats for some time," said Dr. Hal Levison, an Institute scientist in the SwRI Planetary Science Directorate and lead author of the paper. Titled "Growing the Gas Giant Planets by the Gradual Accumulation of Pebbles," the paper is co-authored by SwRI Research Scientist Dr. Katherine Kretke and Dr. Martin Duncan, a professor at Queen's University in Kingston, Ontario.

"It wasn't clear how objects like Jupiter and Saturn could exist at all," continued Levison. New calculations by the team show that the cores of Jupiter and Saturn could form well within the 10-million-year time frame if they grew by gradually accumulating a population of planetary pebbles - icy objects about a foot in diameter. Recent research has shown that gas can play a vital role in increasing the efficiency of accretion. So pebbles entering orbit can spiral onto the protoplanet and assimilate, assisted by a gaseous headwind.

In their article, Levison, Kretke, and Duncan show that pebble accretion can produce the observed structure of the solar system as long as the pebbles formed slowly enough that the growing planets have time to gravitationally interact with one another.

"If the pebbles form too quickly, pebble accretion would lead to the formation of hundreds of icy Earths," said Kretke. "The growing cores need some time to fling their competitors away from the pebbles, effectively starving them. This is why only a couple of gas giants formed."

"As far as I know, this is the first model to reproduce the structure of the outer solar system, with two gas giants, two ice giants (Uranus and Neptune), and a pristine Kuiper belt," says Levison.

"After many years of performing computer simulations of the standard model without success, it is a relief to find a new model that is so successful," adds Duncan.

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User comments

If anyone could tell me, are these accretion processes modeled anywhere in an actual lab? Every time I read an article about planet formation via core accretion, all I see is computer simulations. What are the mathematics from the simulations derived from? Are they based off of legitimate laboratory experiments, and if so, to what scale in size and length have they been pushed to? I'd love sources, if anyone can provide them.

A computer simulation is only a tool for trying out a hypothesis. It doesn't definitively prove the hypothesis but it can show if it's plausible and worth pursuing further. The simulation is only as accurate as the programming, which is also dependent on how well it models and includes all the known inputs. Here's a description of the LIPAD simulation they built:

Another recent test of pebbles is the work on the ejected 5th giant in Nice 2.0 models of the solar system formation.

The giant orbited between Uranus and Neptune, and it gives two kicks. First it makes the Jupiter/Saturn resonance quick, which saves the inner planets from resonance ejection. Then it gives Neptune a quick oribital change too. The latter releases the Kuiper belt kernel, and predicts Neptune's wide orbit and same as the inner system saves the the Kuiper cold population from disturbances.

Here is the cool thing though. The kernel pebble mass is not dense enough to make the cold Kuiper belt objects, except by pebble accretion (which needs ~ 1/100 of other accretion density)!

I would say that the simulations is a lab, a large lab with mostly first order mechanisms. It complements elucidating the mechanisms that can be observed by other means but can't be assembled in toto because of resource constraints (lab space, process time). It also predicts observations.

E.g. you can't make a planet, a star, a cluster, a galaxy, a filament or the cosmic web in the lab. But you can model it and identify the observations that can distinguish between models. If you are lucky, as seems to be the case here, you can reject the other mechanisms. I.e. it isn't certain yet that the other models can't cope, but the pebble accretion mechanism develops fast and seems to solve what earlier models didn't.

"With that data, Johansen said, researchers can compare those properties to the ones we know about in planets in other star systems to see how well they match up.

"Some stories of planet formation are 'avoidable' — that is, there are alternative stories — but the pebble-accretion story is in some form or other likely to be important," David Stevenson, a planetary scientist at the California Institute of Technology in Pasadena, who was uninvolved in the study, told Space.com via email. "This paper shows hope of overcoming some of the previously identified difficulties.""

Thank you both very much for pretty thorough answers, particularly you Torbjorn. In the end though, I'm not particularly satisfied by either of your responses, because they didn't actually provide me with an answer - both of you attempted to justify the validity of using computer simulations and the thinking behind the modelers. Which is a reasonable argument, but it's also not what I asked.

I was asking if these ideas have ever been put to the test in a lab, and no, I don't consider computer simulations genuine laboratory tests. To be frank, I was looking for something more along the lines of this: http://www.scienc...5556.htm

A real-world test, demonstrating how the electro-static force can bring together dusty particles - incidentally, one of Birkeland's ideas. Have there been experiments like this for the core accretion model? I'd love to read about them.

E.g. you can't make a planet, a star, a cluster, a galaxy, a filament or the cosmic web in the lab.

I disagree with this idea, and that approach to laboratory experiment in astronomy. Birkeland was extraordinarily successful at doing just that actually - building massive terella experiments that could replicate a large portion of the solar system's featured, including planets, comets, the Sun, and the aurora. After reading "The Northern Lights" by Jago, Birkeland is right up there with Faraday as a personal hero, and if any line of work demonstrates that you can bring space into the lab, it's his. Kind of disheartening that no one thinks his work is worth interest, or that his methods haven't been adopted.

As you can see I have already responded that I do. So the burden will be onto you to test whether or not that is a critical criteria. I don't think it is, modern modeling has made significant contributions to science such as shown in the pebble models.

As for EU, which you now inject into a discussion of planetary formation, it has long been rejected despite the type of pattern recognition + magic lab work you refer to. E.g. if it looks like 'planets' it is taken for planets, never mind that there is no mechanism involved. It is magic! =D

That seem to be a relevant test of the "laboratory test" criteria. Not only does it not work, it is used by pseudoscientists as a vehicle. I can see how modeling is less popular, since you can many times dissect mechanisms, added to that you need to know what you do and - above all - _quantify_ for testing. As we can see, pseudoscientists fear that!

(1) With respect, I did not inject the EU into this discussion, in terms of the broad, generalized framework you're referring to. I brought up Birkeland: first as a counter-example to your suggestion that we cannot model astronomical bodies/phenomena given his very successful models of the aurora, and second, in relation to the laboratory experiment about electro-static accretion of pebbles. The experiment was successful, and the idea of electro-static forces bringing the particles together was part of Birkeland's ideas. My mention of Birkeland was directly relevant to our conversation.

(2) It is extremely disingenuous to say that the EU, as a broad framework, has "long been rejected" when a very large portion of Birkeland's work, or Alfven's, is *presently* in use by the astrophysical community today - despite the fact that most of the latter's work is published in the IEEE, an electrical engineering journal, and not the primary astrophysical literature. Likewise, it is poor scientific judgment to think that because a hypothesis is initially ruled out, that new evidence can't be discovered that validates the hypothesis. Again, Birkeland and Alfven are perfect examples: the former's ideas were considered *invalid* in 1908. It was fifty years until he was proven right, and only by the launching of spacecrafts into space.

Alfven's ideas of MHD waves was considered a violation of Maxwell's equations, until he demonstrated them in laboratory plasmas. His ideas about critical velocity were considered incorrect until particle rings were discovered orbiting around Uranus, which proved to be a correct prediction. New evidence published on this site throughout this entire year brings at least observation evidence to help support his broad ideas on galaxy formation. An unwillingness to recount new evidence is not a scientific approach.

(3) How you could possibly call Birkeland's work "pattern recognition + magic" is beyond me. The man spent months in Haldde gathering the most extensive magnetic measurements of the aurora of the time, which he then followed up by several more months of *simultaneous* measurements of auroral phenomena across multiple countries including Iceland and Russia. He then took that data, formed a hypothesis, and build a working laboratory model that clearly demonstrated the genuine mechanism of the aurora - cathode rays released from the Sun, drawn to the planet's poles. That is *not* pseudoscience, and it *did* work, so I'm not sure how you can try to backup the point you're asserting. Pseudoscience is the entire British astrophysical community *ignoring* that work, because Kelvin said it was theoretically impossible in 1892. Do people go through university training for astrophysics genuinely not learning this history?

In the end, you still haven't provided me with any laboratory experiments showing me that accretion can work as a viable planet forming mechanism. All that I asked was if you could provide me with more references *exactly* like the one I posted, and it seems like you can't do that.

I think models need to be tested in a lab, and verified through astronomical measurements and observations made in space. That isn't pseudoscience, it's been done very successfully in the past, and I think it's a better approach to conducting astronomy than building computer simulations to test theoretical mathematics. If you disagree, then you're certainly entitled to your opinion.

Another workaround that struggles to explain what we see. Our star, Sol, was in its initial stage a red giant I refer to as the Beetle Sun and had as its entourage the gas giants in close orbits. This is what scientists see in the telescopes. They had even proposed based on what they saw that Jupiter would spiral in. Now they say Jupiter has actually moved out. This is because the Beetle Sun imploded under a magnetic field that created Sol. Sol and the rocky body planets. The exploded planet S W Carey referred to as Aztec was also in Sol's entourage before its demise. The missing mass is in asteroids, meteorites, etc. Possibly the cause of Jupiter's giant red spot.

Here is my argument against your line of reasoning: equations do not model reality, and this general line of thinking, to me, is extremely dangerous. Mathematics is a language, that's all. In the case of physics, and more specifically in engineering, the utility of mathematics is the ability to create a term for every variable being manipulated in a specific experimental setup, to quantify the relationship that manifests between said variables, and the ability to use those relationships to test further hypothesis. That's all - equations are literally nothing but a description a relationship that holds true in a given experiment. You need to be able to *test* that those relationships will work if you're applying them to a secondary hypothesis. Without this, the extrapolated mathematics is just logical guesswork.

In the case of astronomy (in general) the rules say to make progress we use models and observation rather than experiment and prediction to validate.

And on this point, I'm afraid I will always have to disagree, which is why I suppose I don't play by the rules. I don't think we've pushed the limits of our laboratory models far enough, in solar system mechanics, in solar system formation, in galaxy formation. Particularly with the latter - other than Bostick's/Peratt's work, I haven't been able to find anything about spiral-formations in laboratory plasmas, and no one has been able to offer more either. And worse, their work is considered irrelevant, despite new observational evidence that helps support their general ideas.

The limits of Birkeland's terella experiments haven't been reached. Neither have Bostick's or Peratt's. But I don't see anyone working on them, so my interest is in doing exactly that.

Ultimately I understand your point, I just don't think we've gone far enough. And I think, as was the case in the past historically with figures like Birkeland and Alfven, a lot of very good work by plasma physicists regarding electromagnetism is being disregarded on theoretical grounds, not experimental ones.

I'm sure we'll disagree on this, and it's up to me to provide the burden of proof by doing the experiments myself, I understand that. But what I dislike is that the conversation is essentially barred - it's not allowed. Alfven is an extremely successful plasma physicist, and in terms of application to astronomy, he is one of *the most* successful. And yet his criticism of the present astrophysical community and paradigm are thrown out, not discussed, and have been deemed as "case closed" despite everything in just the last few months that supports it. I don't think this is healthy for scientific discussion.

Feel free to move on, I totally understand. My last point if you care to read, but feel no need to respond. Believing the Sun will come up tomorrow isn't logical guesswork, it's a completely reasonable hypothesis made on thousands of years of being a correct prediction. To say that any of our models have been put to *that* kind of test, with a 100% success rate, reveals why the comparison is hyperbole. We're talking about the mechanisms at hand that drive the machine of our solar system, not whether the Sun - or, rather, the Earth, if we're talking about technicalities - will continue to do what it has always done.

To repeat, requiring the impossible is not productive (except where it forces invention).

I require the impossible explicitly to force said invention, so that we have tools to genuinely understand our universe. Birkeland wasn't given his terella, he had to build it himself.

As for your final point about conducting those experiments on Earth, you may have a point there. So, let's conduct them in space. For me, everything comes back down to scale, and the variables at play in the phenomena happening: if we can't use 10E5-6 years to model our experiment, then lets counterbalance that time scale by dramatically reducing the scale of the "terella" that we're trying to build by our accretion experiment. If we can't study the B and E fields at astronomical scales, we need to begin by studying them at laboratory scales, and building up from that. We *need* to get as close to modeling our actual solar system in a lab as possible - which is exactly what Birkeland was working towards before his early passing. Let's build on that! Let's increase the size, the amount of voltage, the quality of the materials in the experiment, the number of terellas, lets let them rotate, lets add in hydrogen plasma to the vacuum, etc.

As you can see I have already responded that I do. So the burden will be onto you to test whether or not that is a critical criteria. I don't think it is, modern modeling has made significant contributions to science such as shown in the pebble models.

I will point out in support of this point that modern modeling has made significant contributions to technology such as development of new computer chips, new aircraft wing and propulsion engineering principles, new submarine propeller engineering principles, new refrigeration and HVAC engineering principles across an entire domain of temperature regulation, new electronic power supply designs, and continuing development of new technological areas of exploration as well as continuing improvements of existing technique.

Am I wrong in thinking that no solar system formation model is ab initio?

Models of solar formation have a great deal of further investigation to yield a fully consistent ab initio model. But we have a fairly consistent understanding of most of the details, considered timewise. As can be seen from this article, and others on this site, this is an area of fairly aggressive investigation at this time. The cosmologists have an ab initio model of the creation of the universe leading to the local characteristics that influenced the creation of the Solar System and so the "pressure is on" (in a friendly manner) for the solar-system-level-specializing astrophysicists now to account for the formation of our Solar System.

There are still, as we see from this article, areas that we will be doing exploration and evaluation of for a long time to come in this area. But that doesn't mean we "know nothing." We know a lot.

@Da Schneib: The extremely large difference here is that all of those models could be tested experimentally in a lab here on Earth, where the products either worked or didn't. The same is not true for astrophysics, and the suggested that they're similar situations is disingenuous, at best. It's completely absurd that I'm catching flack for suggesting that (1) cosmology and astronomy needs more skepticism and less theoretical mathematics when computer simulations with dark matter, black hole formations in the early universe, and accretion models are constantly proved wrong. And they are, because I'm here on this site and many others reading about it. And (2) for "daring" to ask what experimental evidence these models are based on. Is it seriously impossible for anyone to provide me with a single experiment that demonstrates how these models are even physically viable? Are you telling me they're based 100% on theoretical mathematics?

And to try and suggest that modern cosmology has had a similar track record of successful modeling, is also disingenuous. In 2013, the entire model of planetary formation by accretion was called into question: http://www.bbc.co...-problem

I don't know where the hubris in cosmologists come from when they're models are proven wrong vastly more than proven right. Another more recently: the emergent flux model for coronal hole jets has been falsified, and it's been discovered that the mechanism behinds the jets are actually exploding double layers in the filaments that form. https://www.nasa....ure.html

But yeah, god forbid that I simply ask, "hey guys, what experiments did you do to prove this idea - even in concept, if not to scale."

@my2cts: You're here, posting on a science website, trying to dissuade someone from the pursuit of one-day conducting actual experiments, in favor of theoretical mathematics. And yet you pretend to be a champion of the scientific method. What exactly was the point of your comment? To convince me that it can't ever be done, and that I shouldn't try?

I am not wrong to ask you for laboratory evidence for a hypothesis. If accretion models can't be tested by anything other than observation - where it has *failed* as recently as two years ago, then no, I don't consider that a bullet-proof, proven theory. In every other field, from chemistry to biology to electrical engineer, you have to prove your hypothesis in a lab. Astrophysics should not be different, and if you're not up to snuff to push the boundaries and limits, then that's your issue, not mine.

The supporters of big bang mithology claim that these gases are moving with great spead from the center of blast to fictional boudaries of the universe. So how fast moving matter can condense to form stars and planets raises qustions. We know that gravity compared with the main forses in nature have very weak influence on the matter particles. Such kind of sypothesis need strong gravity that should dominate the hypothetical expansion of the matter according to official theory. But if it dominates the expansion, there will be shrinking instead. So this people talk usulay about expansion of matter and space, but when considering the formation of cosmic structures they talk about shrinking. There is no doubt the this people have flexible thinking.

The hypthetical accretion is not responsible for formation of cosmic structures. This structures all are rotating in diffrenet dirctions and the main question is why? And why we observe globular clusters which obviously missed the hypothetical process of accretion.

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